1. Field of the Invention
The present invention relates to a MUSE (Multiple Subnyquiest Sampling Encoding)--NTSC (National Television System Committee) converter capable of transforming a high-vision signal transmitted in the MUSE mode into a signal which can be reproduced on an NTSC monitor.
2. Description of the Background Art
High-vision satellite broadcasting in the MUSE mode is in transition from experimental broadcasting to pilot broadcasting, and the desire to receive high-vision broadcasting at home will probably expand more and more.
The MUSE is not compatible with the current standard NTSC. Thus, a high-vision receiver having a decoder for MUSE signals named "MUSE decoder" built therein is necessary to receive high-vision broadcasting.
On the other hand, there is a strong desire to enjoy high-vision broadcasting on the currently available receiver, and to implement it, a mode converter for varying a MUSE signal into the current NTSC mode, namely, a MUSE-NTSC converter, is necessary and some have been commercially available.
FIG. 27 is a block diagram showing a configuration of the prior art MUSE-NTSC converter. The MUSE-NTSC converter is, for example, disclosed in "MUSE-NTSC CONVERTER," Academic Report of the Television Society 1991 Vol. 45, No. 11, 5-2-3, written by Yoshiki Mizutani, published by the Television Society, Inc.
Referring to FIG. 27, the MUSE-NTSC converter includes an input signal processing circuit 1 for executing input processing on a MUSE signal SM, a time-base transforming circuit 2 for transforming a MUSE mode input signal into an NTSC mode signal on the time-base, a signal separating circuit 3 for separating the NTSC mode signal from the time-base transforming circuit 2 into a luminance signal Y and color difference signals R-Y and B-Y, a Y vertical filter 4 for transforming the luminance signal Y from 1125 scanning lines into 525 scanning lines, a time expander 5 for expanding durations of the color difference signals R-Y and B-Y to four times as long as their initial durations, a C vertical filter 6 for transforming the color difference signals R-Y and B-Y in accord with a treatment of the luminance signal Y transformed by the Y vertical filter 4, and a vertical compressor 7 for further compressing the scanning lines of the transformed luminance signal Y and color difference signals R-Y and B-Y to 2/3. The luminance signal Y and the color difference signals R-Y and B-Y processed in the Y vertical filter 4 and the C vertical filter 6 are directly applied to a first input 8a of a 2-1 selector 8 while the luminance signal Y and the color difference signals R-Y and B-Y processed in the vertical compressor 7 are applied to a second input 8b of the 2-1 selector 8.
The 2-1 selector 8 selects one of the signals received on the first and second inputs 8a and 8b in accordance with a control signal not shown to output it to an image processing circuit 9. The image processing circuit 9 performs various kinds of processing on the signals output from the 2-1 selector 8 to apply the resultant signals to a D-A converter 10. The D-A converter 10 converts a digital signal received from the image processing circuit 9 into an analog signal to output it to the outside and to an NTSC chroma encoder 11. The NTSC chroma encoder 11 produces an NTSC mode chroma signal based upon a signal received from the D-A converter 10. The MUSE-NTSC converter further includes a 16.2 MHz oscillator 12 for producing a 16.2 MHz oscillation signal as a MUSE mode system clock, a 14.742 MHz oscillator 13 for producing a 14.742 MHz oscillation signal as a system clock for both a transformation mode which keeps a roundness ratio on a 16:9 monitor (referred to as "full mode" hereinafter) and a transformation mode which keeps a roundness ratio on a 4:3 monitor by transforming about the whole in the horizontal direction and reducing a transformation rate in the vertical direction to 2/3 (referred to as "wide mode" hereinafter), a 10.08 MHz oscillator 14 for producing a 10.08 MHz oscillation signal as a system clock for a transformation mode which keeps a roundness ratio on a 4:3 monitor by truncating in the horizontal direction (referred to as "zoom mode" hereinafter), and a 3.579545 MHz oscillator 15 for producing a 3.579545 MHz oscillation signal as a system clock used for producing a subcarrier for the NTSC chroma encoder 11.
The oscillation signal from the 16.2 MHz oscillator 12 is applied to the input signal processing circuit 1 and the time-base transforming circuit 2, one of the oscillation signals from the 14.742 MHz oscillator 13 and 10.08 MHz oscillator 14 is applied to the time-base transforming circuit 2, signal separating circuit 3, Y vertical filter 4, time expander 5, C vertical filter 6, vertical compressor 7 and D-A converter 10, and the oscillation signal from the 3.579545 MHz oscillator 15 is applied to the NTSC chroma encoder 11.
An operation of the MUSE-NTSC converter will be described below. The MUSE signal SM, after undergoing various kinds of processing such as deemphasis, control signal detection, PLL and the like in the input processing circuit 1, is transformed on the time-base by the time-base transforming circuit 2.
Specifically, the time-base transforming circuit 2 groups the signal previously processed into odd lines and even lines to separately input them into a time-base transforming memory and receives the 16.2 MHz oscillation signal from the 16.2 MHz oscillator while it also receives the oscillation signal from the 14.742 MHz oscillator 13 via the 2-1 selector 59 in the full mode or the wide mode to transform the system clock from 32.4 MHz to 14.742 MHz. Meanwhile in the zoom mode, the time-base transforming circuit 2 receives the 16.2 MHz oscillation signal while it receives the oscillation signal from the 10.08 MHz oscillator 14 via the 2-1 selector 59 to transform the system clock from 32.4 MHz to 10.08 MHz.
The signal transformed on the time-base is separated into the luminance signal Y and the color difference signals R-Y and B-Y by the signal separating circuit 3; the luminance signal Y is applied to the Y vertical filter 4 while the color difference signals R-Y and B-Y are applied to the time expander 5.
The Y vertical filter 4 reduces the MUSE luminance signal Y in number from 1032 effective scanning lines to 516 scanning lines. In other words, it makes a single scanning line of two scanning lines.
On the other hand, the color difference signals R-Y and B-Y have their respective durations compressed to 1/4 when they are a MUSE signal, and therefore, they are expanded on the time basis four times by the time expander 5. The time-expanded color difference signals are filtered by the C vertical filter 6 so as to control vertical centering with the Y scanning lines. Since the color difference signal is transmitted in alternations of 516 lines, the scanning lines are not transformed but processed with separate filters from line to line so as to control vertical centering of both the color difference signals R-Y and B-Y with the luminance signal Y.
The color difference signals R-Y and B-Y vertically in phase with the luminance signal Y which has undergone scanning transformation are transformed are appropriately selected by the 2-1 selector 8 and coupled to the D-A converter via the image processing circuit 9 in the full mode and the zoom mode while, in the wide mode, these signals, after their effective vertical scanning lines are transformed to 2/3 by the vertical compressor 7, are coupled to the D-A converter via the image processing circuit 9.
For the signals transformed in the full mode, zoom mode or wide mode, the image processing circuit 9 performs various kinds of image processing such as outline modification and the like, and then the D-A converter 10 converts them into analog signals.
The NTSC chroma encoder 11 uses a subcarrier produced based upon the oscillation signal from the 3.579545 MHz oscillator 15 to modulate the analog-transformed color difference signals R-Y and B-Y into NTSC mode chroma signals.
The prior art MUSE-NTSC converter is configured as mentioned above, and usually it requires a first PLL circuit for synchronizing the oscillation signal from the 16.2 MHz oscillator 12 with the oscillation signal from the 14.742 MHz oscillator 13 in phase, a second PLL circuit for synchronizing the oscillation signal from the 16.2 MHz oscillator 12 with the oscillation signal from the 10.08 MHz oscillator 14 in phase, and a third PLL circuit for synchronizing the oscillation signal from the 16.2 MHz oscillator 12 with the oscillation signal from the 3.579545 MHz oscillator 15 in phase, in performing PLL on the MUSE mode system clock and the NTSC mode system clock.
In this way, the prior art MUSE-NTSC converter has a disadvantage that it needs at least three PLL circuits, and this makes the circuitry complicated.
Additionally, the prior art MUSE-NTSC converter, after transforming the color difference signals R-Y and B-Y into the analog signals, requires the NTSC chroma encoder 11 in the following stage for processing the analog signals, and therefore, there arises the problem that a scale of the circuitry is to be increased.